Solid state CCD image sensors often employ a double polysilicon gate structure to form the sensor electrodes. Such a structure has the first polysilicon electrode (poly-1) separated from the second polysilicon electrode (poly-2) by a thin insulating layer of silicon dioxide. Poly-1 is slightly overlapped by poly-2. The systematic variation of the potential applied to these electrodes, referred to as clocking, permits the device to function. In the case of frame transfer CCD image sensors, light passes through the polysilicon electrodes and creates electron hole pairs in the underlying silicon. These electrons are accummulated prior to clocking the polysilicon electrodes to remove the accummulated charge. The polysilicon electrodes, through which light must pass, are not entirely transparent. This lack of transparency results in the reduction of sensitivity and spectral response of the image sensor.
Due to its transparency, it has been recognized that indium tin oxide would be an effective electrode for such a device. The use of an indium tin oxide electrode enhances the blue response and overall sensitivity of a frame transfer image sensor. In fact, it has been recognized that if indium tin oxide were to be used in such a device the effective ASA of the device could be increased by as much as a factor of two. One reason that ITO has not been used on such devices is because it is difficult to pattern such material. Heretofore, the only practical method for etching indium tin oxide has been by immersion in a hot hydroiodic acid solution. Such an acid etches the material isotropically and is not selective to photoresist. These two reasons alone show the difficulties involved in using ITO for microelectronic aPPlications where small features are defined by photoresist lithography.
It has also been recognized that ITO can be used as an antistatic coating on materials such as webs used in the manufacture of photosensitive materials. Thereagain, it is difficult to use such a material because it is not practical to pattern it.